Drilling offshore oil and gas wells includes the use of offshore platforms for the exploitation of undersea petroleum and natural gas deposits. In deep water applications, floating platforms (such as spars, tension leg platforms, extended draft platforms, and semi-submersible platforms) are typically used. One type of offshore platform, a tension leg platform (“TLP”), is a vertically moored floating structure used for offshore oil and gas production. The TLP is permanently moored by groups of tethers, called a tension legs or tendons, that eliminate virtually all vertical motion of the TLP due to wind, waves, and currents. The tendons are maintained in tension at all times by ensuring net positive TLP buoyancy under all environmental conditions. The tendons stiffly restrain the TLP against vertical offset, essentially preventing heave, pitch, and roll, yet they compliantly restrain the TLP against lateral offset, allowing limited surge, sway, and yaw. Another type of platform is a spar, which typically consists of a large-diameter, single vertical cylinder extending into the water and supporting a deck. Spars are moored to the seabed like TLPs, but whereas a TLP has vertical tension tethers, a spar has more conventional mooring lines.
These offshore platforms typically support risers that extend from one or more wellheads or structures on the seabed to a surface wellhead on the platform on the sea surface. The risers connect the subsea well with the platform to protect the fluid integrity of the well and to provide a fluid conduit to and from the wellbore.
The risers that connect the surface wellhead to the subsea wellhead can be thousands of feet long and extremely heavy. To prevent the risers from buckling under their own weight or placing too much stress on the subsea wellhead, upward tension is applied, or the riser is lifted, to relieve a portion of the weight of the riser. Since offshore platforms are subject to motion due to wind, waves, and currents, the risers must be tensioned so as to permit the platform to move relative to the risers. Accordingly, the tensioning mechanism must exert a substantially continuous tension force to the riser within a well-defined range to compensate for the motion of the platform.
An example method of tensioning a riser includes using buoyancy devices to independently support a riser, which allows the platform to move up and down relative to the riser. This isolates the riser from the heave motion of the platform and eliminates any increased riser tension caused by the horizontal offset of the platform in response to the marine environment. This type of riser is referred to as a freestanding riser.
Hydro-pneumatic tensioner systems are another example of a riser tensioning mechanism used to support risers. A plurality of active hydraulic cylinders with pneumatic accumulators is connected between the platform and the riser to provide and maintain the necessary riser tension. Platform responses to environmental conditions that cause changes in riser length relative to the platform are compensated by the tensioning cylinders adjusting for the movement.
With some floating platforms, the pressure control equipment, such as the blow-out preventer and a drilling wellhead, is dry because it is installed at the surface rather than subsea. In some such cases, a nested, dual-riser system may be required where one riser is installed inside another riser. The riser or one of the two risers connecting the subsea wellhead with the surface wellhead may also be held in tension by pulling the riser in tension and then landing the riser in the surface wellhead supported by the platform. The outside of the riser is sealed against the inner diameter of the wellhead using an annular seal. These annular seals however are subject to relative motion between the riser and the wellhead due to the movement of the platform as well as the movement of the equipment above the wellhead. This relative movement presents a potential source of wear on the seal and the seal surfaces.